Ring propellers

ARCHIEF

RING PROPELLERS

By IR L. A. van GUNSTEREN

TO BE READ AT

The St. Lawrence Branch

MONTREAL

September

15th, 1969

and at

The Great Lakes Branch

TORONTO

September 18th, 1969

Lab.,

y.

Scheepsbouwkunde

Technische Hogeschool

Deift

Introduction

A ring propeller is - as the word indicates - a pro-peller with a ring attached to the blade tips. This ring

rotates with the propeller. The ring has an airfoil shape similar to the nozzle of a ducted propeller (Kort-nozzle), so the ring propeller resembles in many aspects this type of propulsion.

Hydrodynamic features differing from the ducted

propeller are:

- The tip clearance is zero.

Consequently there is no efficiency loss due to tip clearance.

The ring rotates with the propeller.

The consequent viscous forces on the rotating

ring produce an extra torque.

In an ideal fluid the ring propeller and the ducted pro-peller with zero clearance behave identically. In the real

fluid the extra torque due to viscous forces on the

ro-tating ring considerably affects the propulsion charac-teristics.

Consequently the design points for optimum efficiency. of both propeller types differ greatly with regard to

dia-meter and rotational speed. The ring propeller should

therefore be considered as an independent type of

pro-pulsion and is not a mere modification of the ducted

propeller.

From a practical point of view the following consider-ations are of interest.

- Sonic

difficulties connected with the ducted

propeller, namely the attachment of the duct

to the hull and the problems with the tip clear-ances (centering of duct and propeller, stiffness of construction, cavitation erosion on the duct),

are circumvented with the application of the

ring propeller.

- The rotating ring affects the moment of inertia.

For this reason a light material should be chosen for the ring.

- The ring propeller has its specific strength

prob-lems. Especially the attachment of the ring to

the blade tips deserves attention.

- The optimum rotational speed at a given diameter is for the ring propeller much lower than for the ducted propeller. This has consequences for the gearing and also affects the cavitation properties in a favourable sense.

* Head. Hydrodynamic Research and Design Department.

Lips Propeller Works NV., Drunen. Holland.

RING PROPELLERS

by:

Ir L. A. van Gunsteren*

The paper discusses the features of propellers with an airfoil shaped ring

attached to the blade tips.

Results of systematic open water test series and as well as experiences

with ring propellers in service are reported.

We may conclude:

The field of application of the ring propeller nay be

expected there where the ducted propeller would

be favourable and the necessity exists to avoid the attachment of the duct to the /lull or tile problems with clearances.

The ring propeller may therefore be attractive for the following ship types:

- large tankers

- ice breakers

- tugboats

- coasters

- fishing vessels

vessels for inland waterways

The ring propeller has already been applied long ago. Figure 1 shows a photograph, published in 1927 in refer-ence [1], of a ring propeller of a twin screw barge sailing on the river Yang Tse Kiang in China.

Figure 1. Ring propeller in service on the Yang-Tse-Kiang in

China 1927.

A Schnitger-propeller is a ring propeller with a ring

at about 0.5 radius. See figure 2. The friction torque

due to the ring is considerably decreased by the reduction of its diameter. The blade tips are at the outside of the ring, so the outer flow field plays a dominant role [2]. The outer flow field induced by a duct vanishes at a relatively small radial distance from the duct.

A ring at half radius seems therefore to be rather

useless. The Schnitger-propeller has indeed proved to

be a failure [3]. A better way for decreasing the

fric-tional torque of the ring is to reduce the rotafric-tional speed

or the diameter of the entire device, which is a basic

point of the ring propellers described in this paper.

Figure 2. Schnitger-Propeller.

Recently a special type of ring propeller has been pro-posed by Sparenbcrg 14]. It has a ring around the blades of which the angles of incidence vary along its

circum-ference. The basic idea is that in this way the strong

vortices of the blades can be evenly spread, resulting in a gain in efficiency. Since the manufacturing of a non symmetrical ring requires an entirely different production method, we restrict ourselves to the ring propeller with a rotational symmetric ring.

The field of application that may be expected indicates that the ring propeller deserves the attention of those who are concerned with ship propulsion. As very little was known about ring propellers, the Netherlands Ship Model Basin and Lips N.y. Propeller Works decided some years

ago to investigate the feasibility of the ring propeller.

Results of this research, pertaining both to the design and the manufacturing of ring propellers, are presented in this paper. In additionsomeexperiences with the ring propel-lers which have been manufactured so far are reported.

Open water test series of the R4-55 ring propeller

Systematic open water test series are essential for

in-sight into hydrodynamic features and for the development of any design method. In this section we present the results of an open water test series of four bladed ring propellers with a blade area ratio of 0.55. The series

have therefore been called R4-55.

The designs for the ring and the propellers have been based on van Manen's experiences with ducted propellers

[5]. It should be noted that the lift of the ring is

de-termined by the advance velocity. The drag and the

pos-sibility of flow separation are depending on the resultant

velocity at the ring surface. The test conditions were

standard-N.S.M.B.--practice (constant rotational speed; Reynolds number above critical value).

The dimensions of the ring are given in figure 3, those of the propellers in figure 4. The open water diagram is

presented in figure 5. A curve fitting procedure has

been applied to the test results with the N.S.M.B. high speed computer. The resulting polynomial representation of the open water diagram is given in table 1.

Figure 4. Particulars of propeller models of R4-55 series.

DRAWN FOR PIO 1.4

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DIAMETER (MODEL) O 240W,,,

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03 04 05 06 0.7 08 09 1.0 11 12 13 14 15 is 'J

Figure 5. Results of open-water tests with ring propeller series R4-55.

16 15 lt. 1OKQ 12 11 to 09 08 07 06 1? 05 04 03 a2 01 o

Table 1 Polynomial representation of test results with ring propeller series R4-55 DIAGRAM RINGPROPELLER SERIES R4-55 N. P 0.5 D zDIAMETER EXCL.RING IN FT. Bp: VA2.5 z 1.08 D Z z 4 N z R.P.M. PzOHP(1HP:76KGM/SEC.) AO/AEz 0.55 A V5 (1-w) IN KNOTS ng Crin9! O z 0.15 7.

0

VA ¿0 0 60 70 80 90 lOO IO 20 30 140 50 160 70 leO 190 200 diagm for the ring propeller series R455.

Figure 7A. Optimum diameter at given intake velocity, power Figure 7B. Optimum rotational speed at given ntake velocity,

and rotational speed. thrust and diameter.

COMPARISON OF THE B4-55 SERIES WITHOUT NOZZLE

AND THE Ka4-55 SERIES IN NOZZLE 19A WITH THE RING PROPELLER SERIES R4-55

r KT = 0.1510 . 0.4963 . - 0.6603 + 0.3549 + 0.6928 . + 0.6869 . - 0.3609 . - 0.02971 . - 0.08256 . + 0.01734 . - 0.004079. + 0.004272. - 0.001306.

i

J J7 P/D (P/D) J5 (PID) J (P/D)2J (P/D)3J3 (P/D)4J (P/D)1 (P/D)J (P/D)4J = K0 0.04880 - 0.02172 + 0.01817 + 0.1097 - 0.01234 - 0.04390 + 0.01724 + 0.002532 + 0.0002465. - 0.001704 - 0.0006699. 1 J2 JO PfD (P/D) J (P/D) J (P/D)IJ (P/D)J (PfD) (PI D)J (P/ D)J I RING PROPELLERS 20 25 30 Figure 6.

Bp-A B S

-

diagram of the series is given in figure

6. Figure 7 shows a comparison with the Wageningen

B4-55 series and the Ka 4-55 series in nozzle 19a. The first step of the design procedure is to select either the optimum diameter D opt at given rotational speed or

the optimum rotational speed n0t at given diameter with prescribed thrust or power, which can be done with figure 8 and 9 and table 2.

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Figure 8. Open-water diagram K0 (J) of R 4-55 series.

The open water diagram has been reprodnced in figure 8 and 9 in which curves of constant efficiency are drawn and the sets of design points with D D0 and those

with n n opt have been indicated. The construction

of these curves is indicated with arrows in figure 8. The D

opt -

lines and the n opt - lines have been

com-puted using the polynomials of table 1.

The diagrams of figure 8 and 9 enable the selection of any optimum design point. For example if it is re-quired to determine the optimum rotational speed n at

given inflow velocity VA, thrust T and diameter D, we can proceed as follows.

We can write for KT

K

T

T T VA2

e2 _D2VA2

n2D2 or: _KT pJ2

VA

T where: nD

-0.0 Kr 0.' K0o a pn2o5 no KTO Vo0, .L..

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Figure 9. Open-water diagram KT (j) of R4-55 series.

Table 2

Functions for determination of optimum design points

Given: Required: Function:

VA) T, D _0pt KT ₌

e

VA2D2) . VA, T, n

_D0t

K1 ₌

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VA, Qn, D _opt KQ =

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n Z o A0 f AE 00,55 5% K0o D n F _{D 1.05 0} K KTO

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ru...

0.2 o .a 0,4 o.) o 0. 0,

The optimum rotational speed is then obtained by:

- calculating the p-value from T, VA and D.

- drawing the curve KT pJ2 in figure 9.

- reading the J-value of the intersection point

of this curve with the - line.

¡fit is desirable to choose the design point in any other point of the curve KT pJ2, then the effect on efficiency can be obtained by comparing this curve with the lines of constant efficiency.

Similar procedures can be followed for the calculation

of the optimum diameter and for the cases where the

power P is given instead of the thrust T. The functions which are to be drawn in the diagrams are given in table

2. The calculation of the optimum diameter at given

intake velocity, power and rotational speed is most easily done with the Bp

-

.- diagram of figure 6.

Comparison with conventional propellers by

tests with a 1800 HP tug model

It may be concluded from comparison with

conven-tional and ducted propellers, as presented in figure 7,

that at given power, rotational speed and intake velocity:

- The optimum diameter of a ring propeller is

about 30% less than that of a conventional one. The pitch ratio is accordingly higher.

- The open water efficiency is considerably lower than both the conventional and the ducted pro-peller.

The reason is obvious. The optimum diameter of the

ringpropeller is lower due to the viscous forces on the

rotating ring. According to simple axial momentum theory the propulsion device with the lower diameter may be expected to have inferior efficiency. A more

reasonable basis for comparison is to assume that both devices fill the same available space in the aperture of the ship, so the outer diameters are equal. The rotational speed is to be optimized for each device, yielding a much

lower optimum rotational speed for the ring propeller than for the conventional propeller. One can derive then from open water diagrams that on this basis the

efficiency of the ring propeller comes close to that of a conventional B-series propeller. In order to investigate

if this is also true for the condition behind the vessel

comparative propulsion and towing tests have been

car-ried out with a 1800 HP tug model, equipped with

B-series propellers and with ring propellers having the same outer diameter. A brief review of the tests will be given in this section referring for details to [6].

The principal dimensions of the tug are given in

table 3.

Table 4

Principal dimensions of

propellers for 1800 HP tug

Type B-series propellers Ring propellers

Determined for full power absorption free running bollard free running bollard

(1692 metric HP at propeller) (200 r.p.m.) (200 r.p.m.) (135 r.p.m.) (135 r.p.m.)

Diameter mmi 2990 2950 2760 2760

Number of blades 4 4 4 4

Pitch at root 1mm] 2243 1773 4755 3887

Pitch at blade tip [mm! 2243 1773 4187 4t18

Pitch at .7R [mml 2243 1773 4408 3792

Blade area ratio 0.535 0.554 0.550 0.550

Table 3

Principal dimensions of 1800 HP tug

Length between perpendiculars 29.50 m

Breath moulded 8.54 m

Draft moulded on F.P. 2.65 m

Draft moulded on A.P. 3.35 m

Displacement 376 m3

Two B-series propellers and two ring propellers of the R4-55 series were selected, capable of absorbing full power in the free running and the bollard condition respectively. The particulars of the propellers are given in table 4. The propellers are shown in the aperture of the tug in figure

10 and 11. The results of the propulsion and towing

tests arc presented in figure 12 for the propellers designed for the free running condition and in figure 13 for those

designed for the bollard condition.

In figure 14 the

pull/power ratio is plotted as a function of speed for the four propellers. Some of the variations in pull per

HP can be explained by the differences in propeller load-ing. The device with the highest thrust at a certain speed

may be expected to show the lowest pull/power ratio.

Over most of the speed range the ring propellers operate at larger loading than the conventional propellers.

The following conclusions can be drawn:

- A slightly higher speed may be expected with

the conventional propeller designed for free run-fling than with the comparable ring propeller.

- A considerably higher speed may be expected

with the ring propeller designed for the bollard condition than with the comparable conventional propeller.

- Higher pulls can be achieved with the ring

pro-pellers than with the conventional propro-pellers. In the cases where the propellers were designed for free running the bollard pull of the ring propeller is about 22 percent higher than that of the

com-parable conventional propeller. When the pro-pellers are designed for the bollard condition

the gain in pull is about 6 percent.

- The ring propeller is more suited to absorb the

available power in off-design conditions than the conventional one. The differences in pull per HP

are small, particularly when the effect of

pro-peller loading is allowed for.

PROPELLER FOR FREE RUNNING PROP. FOR BOLLARD CONDITION

Figure 10. Conventional propellers in aperture of tug.

RINGPROPELLER FOR FREE RUNNING RINGPROP. FOR BOLLARD CONDITION

Figure 12. Towing and propulsive performance of ring pro-peller and 8-serie propeller destined for free

running. Figure 11. Ring propellers in aperture of tug.

15

10

Figure 13. Towing and propulsive performance of ring

pro-peller and B-serie propro-peller destined for bollard

condition.

N

SHIPSPEED (KNOTS)

Figure 14. Pull/power of ring propellers and B-serie

propellers.

Table 5 Principal dimensions of 850 HP coaster

PULL (N (<G

D HPr,,jC

Length between perpendiculars Breath moulded

Depth to upper deck

Draft moulded Displacement Speed 59.70 m 11.78 m 5.79 m 3.58 m 1500 rn 11 knots

Full scale experience

Five ring propellers have been manufactured so far, and were fitted to two motoryachts, two tugboats and a coaster. These propellers were for existing vessels and

therefore, contrary to the cases of the last section, the design had to be based on given power and rotational

speed. If the original propeller is operating at optimum diameter, then the optimum diameter of the ring propeller is much smaller and the open water efficiency accordingly lower. See figure 7. If, however, the diameter of the original propeller is restricted, then it may he that the

ring propeller still can operate at optimum diameter and the open water efficiency may be the same or even high-er than that of the conventional propellhigh-er. For instance,

if we assume both devices to possess the same outer

diameter and the ring propeller is operating at optimum diameter, then a slight gain in open water efficiency in

favour of the ring propeller can be deducted from the

open water diagrams. One of the tugs was operating at

restricted diameter and in that case a favourable open

water efficiency could be predicted. In the other cases

a loss in open water efficiency was expected. It turned out, however, that in those cases the overall efficiency was not impaired which can be explained by the favour-able effect of the smaller diameter of the ring propeller on the hull efficiency.

The full-scale designs were slightly different from the series propellers. The outside of the ring-profile was

approximated by a somewhat straighter line than that of the ring of the series. The radial pitch distribution was chosen to be constant. The largest of the ring propellers

produced, that of the coaster, will now be discussed in more detail.

The main dimensions of the coaster are given in

table 5.

The original propeller is presented in figure 15, the

ring propeller in figure 16. Both propellers were designed

for 850 HP at 250 r.p.m. and a shipspeed of 11 knots

(wake fraction about 0.32). As both diameters are

Figure 15. Conventional propeller of 850 HP coaster.

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DIAMETER

2300 r,,,,, PITCHICONSTANT) 1000,,,, NUMBER OF BLADES 4 B. A. 8. 0.479 MATERIAL CUNIAL-BRON WEIDHI 1000 4 002 880 gn,2

ENGINE OUTPUT BOO HF 200 R

RING PROPELLERS

RINOPROPELLER (FULL POWER FREE RUNNING RI NOPROPELLER (FULL POWER BOLLARD I BSERIES PROP. (FULL POW E R FREE RUNNING I

B - SERIES PROP, ¿ FULL POW E R BOLLARD

h 6 8 10 12

£

L

=:

5.1 R

ça.

DIAMETER iNTO ,. PITCH (CONSTANT) 2A00 .,.,

NUMBER OF BLADES B AR. 0,55 MATERIAL LIMA.BRONZE WEIGHT )pp.g) 770 k9 502 )prop.LI .r,rn ç 705 kgZ ENGINE OUTPUT 850 H P 250 RPM,

Figure 16. Ring propeller of 850 HP coaster.

optimum, the diameter of the ring propeller is consider-ably smaller than that of the original propeller. The ring was made of a reinforced plastic, having a specific gravity

of 2.1. It turned out that both the weight and the moment of intertia of the ring propeller could be kept below the values of the conventional propeller. See figure 1 5 and 16. The ring was attached to the propeller with dowels as indicated in figure 17. A photograph of the propeller after manufacturing is shown in figure 18. The ring

pro-peller in the aperture of the ship in dock is shown in figure 19.

Figure 19. Ring propeller in the aperture of a coaster.

RING PROPELLERS

lo DOWELS

CUNIAL Øl2mm

ARALOITE BLADETIP

Figure 17. Attachment of the ring to the blades.

Figure 18. Ring propeller after manufacturing in the workshop.

After the propeller had been in service for half a year a heavy pounding was noticed during nlanoeuvring close

to the port of Lissabon and upon investigation it was

found that the ring had disappeared. The propeller ap-peared to be badly damaged, as can be seen from figure 20. lt may be concluded from the severe deformations at the leading and trailing edges of the blades that a very heavy object must have struck the propeller. Although a conventional propeller probably would also have had severe damage under such circumstances, an attempt will be made to improve the propeller with a more solid at-tachment of the ring to the blades. See figure 21.

The ship proceeded on its voyage with a small loss

in speed and conditions regarding vibration were similar to conditions as noted with its original conventional pro. peller.

Figure 21. Improved attachment of the ring to the blades.

Before the unfortunate loss of the ring the ring propel-ler's performance was excellent. After the fitting of the ring propeller the ship out ran the two sister ships which had been in drydock for hull cleaning at the same time. The better propulsion performance of the ring propeller was eonfirnicd by the reports of the chief engineer, from which report can be deducted that the overall efficiency was approximately 5% higher with the ring propeller than with the original propeller. This seems to be an amazing conclusion as the open water efficiency of the ring propel. 1er is lower. Apparently there must have been a very

favourable effect on the hull efficiency. lt is the intention to investigate if this effect can be confirmed with model

tests.

Figure 20. Damaged blade of the ring propeller of the

850 HP coaster.

In addition the following observations were made by the chief engineer of the vessel:

- The engine ran quietly.

- Practically no vibrations were observed. The reduction in vibration level was the most signi-ficant within the range of resonance.

- Less vibrations than before were present at astern thrust.

- In heavy weather the rotational speed could

practically be maintained resulting in about 10

r.p.m. more than with the original propeller in

similar conditions.

- Operating in fog or approaching a harbour is

more difficult with the ring propeller, as with a rotational speed reduced to 1 80 r.p.m. the ship still sailed over 10 knots.

- The head reach at a stopping manoeuvre was

about twice as large with the ring propellers than with the original propeller.

- The response of the rudder was better with the

ring propeller.

These features are well known from experiences with ducted propellers so we shall not discuss theni here.

The ring propellers of the tugboats and rnotoryachts showed equally satisfactory performance in service. Cost estimates of the ring propellers indicate that, if power and r.p.m. are kept the same, the prices of conventional and ring propellers are of the same order. In other words the

ring is paid by the saving in propeller material due to

the smaller diameter.

Concluding remarks

The open water series of ring propellers presented in

this paper are to be regarded as a first try. It is most likely that the open water efficiency can be improved with

further research. The ring could be made thinner and the loading on the ring could be increased. The length

of the ring may be expected to be in the optimum range. Some ring propellers with a ring.length of 0.26 D have been tested which yielded lower efficiencies than with the

R 4-55 series.

The intention is to carry out further research in the

following stages:

i. Improvement of the connection between the ring and the blade tips.

A comparative test with a coaster model in order to investigate the effect on hull efficiency.

Open water tests with systematic series with a

modified ring.

Tests with the modified series behind a ship model

for investigating the hull efficiency and relative rotative coefficient.

Fourquadrant measurements for investigating stop-ping performance.

At the same time information about the performance of the ring propellers in service will be collected.

The design of ring propellers can be done by modifying

any design procedure for ducted propellers

[7]. A

correction for the viscous forces on the ring has to be introduced and the forces transmitted by the ring onto the blades have to be incorporated in the strength

cal-culation.

At last we review in table 6 the features of ringpropel-1ers compared with conventional and ducted propellers. It may be concluded that the ring propeller is a promising device for a specific field of application.

Acknowledgement

This paper contains the results of the efforts of many.

The author has only been given the honour to present

this work. Prof. Dr Ir J. D. van Manen, Director of the Netherlands Ship Model Basin, initiated the project and gave stimulating guidance in every stage, especially with the design of the systematic test series. Useful work re-garding the evaluation of the test data and the develop-ment of design procedures has been done by the author's co-worker Mr. W. A. Arnoldus and particularly during his stay at the N.S.M.B.

Table 6 Features of ring propellers compared with conventional and ducted propellers

ring propellers

compared with advantages disadvantages

Conventional propellers

- superior in off-design conditions

- good efficiency if the conventional

propeller operates at restricted diameter

- protection of the propeller by the

ring (manoeuvring in ice) - favourable cavitation properties - reduced noise level (warships)

- less vibrations

- improved rudder response

- for a good open water efficiency

the rotational speed has to be

re-duced with consequences for

gear-ing and shaftgear-ing

- difficulties with maintaining low

cruising speeds

long head reach

Ducted

propellers

- no clearances problems (centering

of propeller and duct, cavitation

erosion)

- no attachment to the hull - lower price

- lower efficiency

- thrust of the ring

is transmitted

through blades and shafting to the thrustbloc

The manufacturing of the ring was carried out by Mr. J. A. van Beckhoven. The experiences with the ring pro-pellers in service were made possible by Mr. P. J. Kers of the Royal Netherlands Steamship Company. Mr. M.

M. H. Lips, President Director of Lips N.y., approved

the necessary financial support.

It is a great pleasure to the author to acknowledge

these contributions and also those which were not men-tioned explicitly.

References

Il W. Teubert: "Schiffahrts- und schiffbautcchnische Eindrücke

meiner Weltreise", Jahrbuch der Schiffbautechnischen

Gesellschaft, vol. 27, 1927.

[21 H. Amtsberg: "Grundsätzliches zum Ringpropelier", Hansa,

vol. 87, 1950.

[3 J. D. van Manen: "Resultaten van voortgezette proefnemin-gen met de Schnitger-propeller", N.S.M.B.--publication no. 101, Schip en Werf, vol. 19, 1952.

[41 J. A. Sparenberg: "On optimum propellers with a duct of

finite length", Mathematisch lnstituut Universiteit Gronrn-gen, report TW-54, 1968.

[51 J. D. van Manen and A. Superina: "The design of screw

propellers in nozzles", N,S.M.B.publicaliori no. 137,

International Shipbuilding Progress, vol. 6, 1959.

[61 W. H. auf'm Keller: "Comparative tests with B-series and

Ringpropellers", NS. MBreport no. 66-047-D.W.T.,

1966.

171 J. D. van Manen and M. W. C. Oosterveld: "Analysis of

Ducted Propeller Design", Trans. S.N.A.M.E., 1966.